bug #20418 [Form][DX] FileType "multiple" fixes (yceruto)

This PR was squashed before being merged into the 2.7 branch (closes #20418).

Discussion
----------

[Form][DX] FileType "multiple" fixes

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/issues/12547
| License       | MIT
| Doc PR        | -

# (1st) Derive "data_class" option from passed "multiple" option

Information
-------------

Following this tutorial ["How to Upload Files"][1] but storing many `brochures` instead of one, i.e.:

```php
// src/AppBundle/Entity/Product.php

class Product
{
    /**
     * @var string[]
     *
     * @ORM\Column(type="array")
     */
    private $brochures;

    //...
}
```

```php
//src/AppBundle/Form/ProductType.php

$builder->add('brochures', FileType::class, array(
    'label' => 'Brochures (PDF files)',
    'multiple' => true,
));
```

The Problem
--------------

I found a pain point here when the form is loaded again after save some brochures (Exception):

> The form's view data is expected to be an instance of class Symfony\Component\HttpFoundation\File\File, but is a(n) array. You can avoid this error by setting the "data_class" option to null or by adding a view transformer that transforms a(n) array to an instance of Symfony\Component\HttpFoundation\File\File.

The message is very clear, but counter-intuitive in this case, because the form field (`FileType`) was configured with `multiple = true`, so IMHO it shouldn't expect a `File` instance but an array of them at all events.

The PR's effect
---------------

**Before:**

```php
$form = $this->createFormBuilder($product)
    ->add('brochures', FileType::class, [
        'multiple' => true,
	'data_class' => null, // <---- mandatory
    ])
    ->getForm();
```

**After:**

```php
$form = $this->createFormBuilder($product)
    ->add('brochures', FileType::class, [
        'multiple' => true,
    ])
    ->getForm();
```

# (2nd) Return empty `array()` at submit no file

Information
-------------

Based on the same information before, but adding some constraints:

```php
// src/AppBundle/Entity/Product.php

use Symfony\Component\Validator\Constraints as Assert;

class Product
{
    /**
     * @var string[]
     *
     * @ORM\Column(type="array")
     *
     * @Assert\Count(min="1") // or @Assert\NotBlank()
     * @Assert\All({
     *     @Assert\File(mimeTypes = {"application/pdf", "application/x-pdf"})
     * })
     *
     */
    private $brochures;
}
```

This should require at least one file to be stored.

The Problem
--------------

But, when no file is uploaded at submit the form, it's valid completely. The submitted data for this field was `array(null)` so the constraints pass without any problem:

* `@Assert\Count(min="1")` pass! because contains at least one element (No matter what)
* `@Assert\NotBlank()` it could pass! because no `false` and no `empty()`
* `@Assert\File()` pass! because the element is `null`

Apart from that really we expecting an empty array instead.

The PR's effect
----------------

**Before:**

```php
// src/AppBundle/Entity/Product.php

use Symfony\Component\Validator\Constraints as Assert;

class Product
{
    /**
     * @var string[]
     *
     * @ORM\Column(type="array")
     *
     * @Assert\All({
     *     @Assert\NotBlank,
     *     @Assert\File(mimeTypes = {"application/pdf", "application/x-pdf"})
     * })
     *
     */
    private $brochures;
}
```

**After:**

```php
// src/AppBundle/Entity/Product.php

use Symfony\Component\Validator\Constraints as Assert;

class Product
{
    /**
     * @var string[]
     *
     * @ORM\Column(type="array")
     *
     * @Assert\Count(min="1") // or @Assert\NotBlank
     * @Assert\All({
     *     @Assert\File(mimeTypes = {"application/pdf", "application/x-pdf"})
     * })
     *
     */
    private $brochures;
}
```

  [1]: http://symfony.com/doc/current/controller/upload_file.html

Commits
-------

36b7ba6 [Form][DX] FileType "multiple" fixes

src/Symfony/Component/Form/Extension/Core/Type/FileType.php

@@ -12,12 +12,37 @@
 namespace Symfony\Component\Form\Extension\Core\Type;
 
 use Symfony\Component\Form\AbstractType;
+use Symfony\Component\Form\FormBuilderInterface;
+use Symfony\Component\Form\FormEvent;
+use Symfony\Component\Form\FormEvents;
 use Symfony\Component\Form\FormInterface;
 use Symfony\Component\Form\FormView;
+use Symfony\Component\OptionsResolver\Options;
 use Symfony\Component\OptionsResolver\OptionsResolver;
 
 class FileType extends AbstractType
 {
+    /**
+     * {@inheritdoc}
+     */
+    public function buildForm(FormBuilderInterface $builder, array $options)
+    {
+        if ($options['multiple']) {
+            $builder->addEventListener(FormEvents::PRE_SUBMIT, function (FormEvent $event) {
+                $form = $event->getForm();
+                $data = $event->getData();
+
+                // submitted data for an input file (not required) without choosing any file
+                if (array(null) === $data) {
+                    $emptyData = $form->getConfig()->getEmptyData();
+
+                    $data = is_callable($emptyData) ? call_user_func($emptyData, $form, $data) : $emptyData;
+                    $event->setData($data);
+                }
+            });
+        }
+    }
+
     /**
      * {@inheritdoc}
      */
@@ -39,9 +64,7 @@ class FileType extends AbstractType
      */
     public function finishView(FormView $view, FormInterface $form, array $options)
     {
-        $view
-            ->vars['multipart'] = true
-        ;
+        $view->vars['multipart'] = true;
     }
 
     /**
@@ -49,10 +72,18 @@ class FileType extends AbstractType
      */
     public function configureOptions(OptionsResolver $resolver)
     {
+        $dataClass = function (Options $options) {
+            return $options['multiple'] ? null : 'Symfony\Component\HttpFoundation\File\File';
+        };
+
+        $emptyData = function (Options $options) {
+            return $options['multiple'] ? array() : null;
+        };
+
         $resolver->setDefaults(array(
             'compound' => false,
-            'data_class' => 'Symfony\Component\HttpFoundation\File\File',
-            'empty_data' => null,
+            'data_class' => $dataClass,
+            'empty_data' => $emptyData,
             'multiple' => false,
         ));
     }

src/Symfony/Component/Form/Tests/Extension/Core/Type/FileTypeTest.php

@@ -44,6 +44,33 @@ class FileTypeTest extends \Symfony\Component\Form\Test\TypeTestCase
         $this->assertNull($form->getData());
     }
 
+    public function testSubmitEmptyMultiple()
+    {
+        $form = $this->factory->createBuilder('file', null, array(
+            'multiple' => true,
+        ))->getForm();
+
+        // submitted data when an input file is uploaded without choosing any file
+        $form->submit(array(null));
+
+        $this->assertSame(array(), $form->getData());
+    }
+
+    public function testSetDataMultiple()
+    {
+        $form = $this->factory->createBuilder('file', null, array(
+            'multiple' => true,
+        ))->getForm();
+
+        $data = array(
+            $this->createUploadedFileMock('abcdef', 'first.jpg', true),
+            $this->createUploadedFileMock('zyxwvu', 'second.jpg', true),
+        );
+
+        $form->setData($data);
+        $this->assertSame($data, $form->getData());
+    }
+
     public function testSubmitMultiple()
     {
         $form = $this->factory->createBuilder('file', null, array(